1. Field of the Invention
The present invention relates to a congestion processing mode and a circuit thereof used when congestion occurs in a frame relay switching network. More specifically, it relates to a mode and a circuit, as a means of recovery or avoidance of congestion, for performing processing for discarding from the inside of the switching network those frames which can not be completely transferred since the frames in the frame relay switching network are congested or processing for performing notification requesting the reduction of the amount of communication data to related subscribers since the network is in a congested state.
As a technique for data switching between subscribers, there has been conventionally adopted a line switching mode or a packet switching mode.
Recently, however, so as to cope with the demands for high speed data switching, a frame relay switching mode, which is a further revision of the packet switching mode, has been proposed and is now rapidly spreading in use. This frame relay switching mode is a mode in which protocols of flow control and transmission confirmation among the protocols positioned as the layer-2 functions in an open system interface (OSI), which have been carried out for every relay link in the past, are eliminated. Processing is carried out by protocols between subscribers (end - end) in place of this according to need, whereby a great simplification of the protocols in data switching is achieved and high speed data switching is realized. Note that, this mode is being standardized in the CCITT, etc. at present.
As described above, in the frame relay switching mode, flow control is not performed for each link passed through, and therefore when assuming that for example a large number of subscribers simultaneously transmit data (frames) to the switching network, the transmission delay inside the switching network becomes terrible and in the worst case forces some frames to be discarded. In the protocol of the frame relay, as a means of avoiding such a situation, as a rule, it is notified in advance to the related subscribers (line users) that the situation is reaching a congested state, so the subscribers received this notification keep down the amount of the communication data. This discard operation must be carried out from the frames having the lowest importance with a high priority. For this purpose, in a header portion of the frame, information indicating that the data may be discarded is written as a discard eligibility indicator (DE) bit. Also, sometimes it becomes necessary to discard the data exceeding the amount of communication data preliminarily set. At the time of congestion, the discard operation is begun with these frames, whereby the deterioration of the quality of the data is suppressed. Note that, this mechanism has been described in CCITT recommendations I.370.
2. Description of the Related Art
In the control performed with respect to congestion inside a frame relay switching network, where it is judged that congestion occurs by monitoring the congestion of the resources of the network such as the relay lines and exchanges with respect to all assigned connections using the related resources, for example, the subscribers of a permanent virtual circuit (PVC), it is notified in advance that there is a possibility that frames will be discarded and then the discard operation of frames is executed. At this time, as an example of the mechanism of detection of the congestion, there can be mentioned a means for watching the transmission queue of frames mounted to the destination line (usually the relay line) of the frames and, when detecting that the length of the queue becomes greater, judging that congestion occurs.
When the frame discard operation mentioned above is carried out only based on the state of congestion of the resources, for example, where a very small group of the subscribing PVC's (assigned connections) transfer a large amount of frames and occupy a large part of the resources, that is, bands, of the related relay line, other subscribing PVC's which try to use this relay line suffer from a great transfer delay of the frames find their frames discarded even if they try to communicate within a range of the originally committed terms of subscription of communication (committed information rate, committed burst amount, etc.), so the quality of the communication service with respect to these other subscribing PVC's is unjustly lowered.
As processing for eliminating such an inconvenience, it is possible to set the aforesaid DE bits in accordance with the above-described terms of subscription of communication and discard the frames while referring to the DE bits and, therefore, avoid the deterioration of the quality of the communication service described above. However, when doing this, irrespective of the degree of the above-described congestion, if a communication even slightly exceeds the committed terms of subscription of communication, the user will be immediately exposed to the possibility of frame discard, resulting in the problem that the advantage of the frame relay exchange mode that the resources of the switching network are effectively utilized by allowing communication to a certain extent even if the communication exceeds the committed terms of subscription of communication, is decreased or cancelled out.
Also, so as to avoid other PVC's being affected due to the increase of the amount of communication data of a certain specific PVC (assigned connection), consideration may be given to a procedure in which buffers are provided independent for every PVC, but there are problems in that not only does the management of these buffers become complex, but also an excessive amount of memories for forming these buffers become necessary.